Creating a 100 mpg car is actually quite easy. Engineers have designed vehicles that were capable of several times that amount. The problem is that those vehicles could barely qualify as "cars," much less a vehicle that can take a family of four to the movies. Many of these vehicles also use very expensive materials. That's the real challenge: creating a consumer-friendly car that's attractive, reasonably priced and capable of going 100 miles on a gallon of gas.

To that end, the X Prize Foundation recently announced the Automotive X Prize. You may remember the Ansari X Prize, won by the SpaceShipOne team. The Automotive X Prize is a contest to build a "production-ready" four-passenger car that could be profitable to produce on a wide scale [Source: Edmunds]. The car has to be capable of 100 mpg and has to win a series of races. The prize is expected to be up to $25 million, though some participants, especially those backed by corporations or prominent entrepreneurs may spend even more than that on their contest entries. The Automotive X Prize allows participants to use several different types of fuel, so some teams might be using natural gas, ethanol, diesel or electricity.

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Given the designs that are already out there and Google.org's success with its Prius plug-in hybrids, multiple teams could feasibly break the 100 mpg threshold. To do it, they'll have to improve fuel efficiency, which means cutting weight. Strong, lightweight materials such as aluminum, carbon fiber and magnesium can lighten the car's body, electronics and wheels. (Some of these materials, like carbon fiber, can be expensive, so the team has to remember that this car must be profitable to produce and market.) Glass actually adds a lot of weight to a car. Types of lightweight glass and composites are being developed, some of which might make their way into Automotive X Prize cars.

Resistance is another key feature for fuel efficiency. Aerodynamic design and low-resistance tires can decrease drag, though those tires can decrease handling. Finally, improved insulation would reduce the need for heating and cooling systems, which would cut down on weight and save gas, though the Automotive X Prize requires entrants to have some sort of air-conditioning system as well as a stereo.

The Automotive X Prize will release its final rules this summer, designs will be reviewed next year and the final batch of races will likely be in mid-2009. While you're waiting for your 100 mpg car, keep your eye on the efforts of Google, the EPA's hydraulic hybrid, the Tesla Roadster and the many researchers, entrepreneurs and amateurs who are looking to create the next generation of ultra-high-mileage cars. It seems Detroit has caught up as well: Google reports that no less than seven automakers are researching plug-in hybrids.

For more information about fuel efficiency, hybrid cars and related topics, check out the links on the next page.